THE INFLUENCE OF COBALT ION CONCENTRATION ON THE DEGRADATION OF METHYL BETA-D-GLUCOPYRANOSIDE IN OXYGEN-ALKALI.

摘要

The influence of cobalt concentration (0.01, 0.05, and 0.25 mM CoSO(,4)(.)7H(,2)O) on the degradation of 95.5 mM methyl (beta)-D-glucopyranoside (MBG) in an O(,2)-alkali system was studied in a teflon-lined reactor at 120(DEGREES)C using 1.25 M NaOH and 0.68 MPa oxygen pressure. The degradations were followed by quantitative gas-liquid chromatography and were analyzed for both degradation rate and product appearance.; The rate of MBG degradation was dependent on cobalt ion solubility. Increasing the concentration of cobalt(II) initially present in reaction solutions from 0.00 to 0.01 to 0.05 mM decreased the time required for 50% MBG degradation from 12 to 6 to just under 2 hours. Attempts to prepare a 0.25 mM cobalt(II) reaction solution by addition of aqueous cobalt(II) to an alkaline stock solution prior to addition of solid MBG (the procedure used in the 0.01 and 0.05 mM reactions) resulted in the formation of a highly adsorptive cobaltous precipitate which was capable of removing otherwise soluble cobalt(II) from solution. Reactions conducted with solutions prepared in this manner resulted in MBG degradation rates which were essentially the same as in uncatalyzed reactions.; The initial rate of MBG degradation at the 0.25 mM added cobalt(II) level was substantially increased if silicate anions were initially present in stock alkaline solutions, or if the reaction solution preparation procedure was reversed so that aqueous cobalt was added to an alkaline MBG solution. In these instances cobalt(II) at the 0.25 mM addition level was stabilized in alkaline solution and no precipitation occurred until after reaction initiation by oxygen pressurization. Visible spectral studies showed that silicates and selected polyols (MBG, sorbitol, glycerol, and ethylene glycol) stabilized cobalt(II) in strongly alkaline (pH 14) solutions.; Cobalt(III) gradually precipitated from solution during degradations which were initially 0.25 mM in soluble cobalt(II). The precipitate formed under degradation conditions was highly adsorptive, and removed otherwise soluble cobalt ions from solution. The precipitation of cobalt(III) from reaction solutions coincided with a rapid decline in the rate of MBG degradation.; The presence of either soluble or precipitated cobalt species in reaction^solutions had only a minor influence on the pattern of acidic products^formed during MBG degradation. The extent of MBG degradation^occurring through glycosidic bond cleavage (as detected through^methanol formation) was the same (ca. 70%) both in the presence and^absence of added cobalt. At later reaction times (40-60% reaction) the^presence of soluble cobalt in reaction solutions increased the^concentrations of dicarboxylic acids relative to uncatalyzed degradations.^A mechanism involving direct oxidation of MBG by cobalt(III) is proposedto account for the enhanced formation of dicarboxylic acids. Catalysis bysoluble cobalt ions is attributed to a catalytic oxidation-reduction cyclebetween cobalt(II) and cobalt(III). It is proposed that the initial oxidationof cobalt(II) to cobalt(III) by oxygen produces superoxide radical (O(,2)),(' )and that cobalt(III) and superoxide radical promote oxidative MBG degradation reactions.